The present invention relates to a system for forming an image of a small light source, such as a short arc lamp, which emits right along a first path of rays. In particular, the system has a relative high aperture which amounts preferably to at least 0.5, but can be much higher. This system uses a spherical, concave reflector receiving said light along the above-mentioned first path of rays and having a concave reflecting surface for reflecting the light received along a second path of rays, said spherical concave reflector means defining an axis of symmetry, the light source being arranged asymmetrically with respect to said axis of symmetry. If in the context of this specification a xe2x80x9creflectorxe2x80x9d is mentioned, it will normally be a more or less equally bent surface, although old Archimedes is told have it formed from individual partial surfaces positioned to form approximately a concave surface (which would also be possible within the scope of the invention).
Such a system is known, for example, from GB-A-1,114,326 for forming an image onto the entrance surface of a light guide. However, due to the relative large angular distance of the light source from the axis of symmetry of the spherical, concave reflector, optical aberrations will occur in the image formed onto the entrance surface of the light guide. The astigmatism occurring with the reflection from the spherical, concave mirror reduces significantly the efficiency of the illuminating system.
To achieve an efficiency of a system for forming an image of a small light source as high as possible, such as with an electric arc lamp of 1 to 2 mm arc length onto an entrance surface of the light guide of about 1to 2 mm in diameter, it is important to ensure accordingly a magnification of about 1:1. This makes it necessary to avoid optical image distortions.
In EP-A0 251 623, an illumination system is disclosed using a spherical concave mirror and an optical correction element in the above identified second path of rays, i.e. between the spherical concave mirror and the entrance surface of the light guide, for increasing the efficiency and for reducing image distortions. This document suggests to arrange the light source as close to the axis of symmetry of the spherical concave mirror in order to keep image distortions principally small. However, this requires that the entrance surface of the light guide is equally close to the axis of symmetry. Therefore, this design has the great disadvantage that the heat emitted from the light source, due to the location of the light guide in close proximity to the light source, will destroy the light guide. Such light guides, independent of what they are made, be it an individual light fiber or a light fiber bundle, are relative expensive. When the system is used, for example, for illumination in medical endoscopes, a high reliability is a pre-condition.
It is an object of the present invention to avoid these disadvantages of the prior art. It is a further object to provide a system which is able to have the light guide in a sufficient distance from the light source so as to avoid its destruction. Another object of the invention is to provide a highly reliable system of a high efficiency, avoiding image distortions or aberrations.
These objects are achieved, according to the invention by providing at least one cylindrical lens of positive diffraction in the first path of rays, i.e. between the light source and the spherical concave reflector.
In this way, a well corrected image of the light source onto the entrance surface of the light guide will be achieved, thus rendering the efficiency very high, in spite of a high numerical aperture and even with a relative large angular distance of the light source and its first path of rays from the axis of symmetry of the reflector. Therefore, the light guide will also have a large angular distance from the axis of symmetry and, thus, from the hot light source and can no longer be affected.
Although the cylindrical lens may be aspheric, it is simpler if it is a circular cylindrical lens.
According to a preferred embodiment, the cylinder axis of the cylindrical lens is arranged perpendicularly to the first path of rays and in a meridian plane which is defined by the first path of rays and the axis of symmetry of the reflector. In this manner, any astigmatism is corrected in an optimal fashion with at low expenses and in a simple way.
According to a further characteristic of the invention, the cylindrical lens is coated with a heat protection coating, preferably a dielectric one. This is of advantage, because in this way, heat radiation of the light source emitted towards the reflector is significantly reduced.
In a further embodiment of the invention a second spherical concave reflector is provided at the side averted from the cylindrical lens to form a directional light source that defines a beam direction, the second spherical concave reflector having an axis of symmetry coincident with the above-mentioned first path of rays. In this way, the efficiency of the system is further increased by a simple and inexpensive optical component.
According to a preferred embodiment of the invention, the first and/or second reflector comprise a dielectric coating so as to form a cold light mirror in order to keep heat radiation emitted from the light source, despite the high efficiency of the system, to a high extent away from the light guide.